Special emphasis has been given to development of methodologies and study of cell proliferation during the past year. Immunohistochemical procedures for staining S-phase cells labeled with bromodeoxyuridine have been optimized and are in routine use in rats and mice. We have established an immunohistochemical methodology to identifying cells expressing proliferating cell nuclear antigen (PCNA) and have successfully applied this procedure to archival specimens that were fixed in formalin and embedded in paraffin. This allows us to take advantage of the National Toxicology Program tissue archives and the thousands of samples available from past studies. A computer assisted program has been developed to automate counting of cell proliferation and simultaneously obtain ploidy data on rodent liver samples. Standard morphometric procedures are being used to study the progressive development of preneoplastic lesions in mouse liver. This work will continue for the next two years.